The use of thin films of superconductive material, in particular of so-called "high critical temperature" superconductive material, is likely to see major development, in particular for microwave applications in electronic instrumentation, etc. . . .
In most applications, it is almost always essential to obtain monocrystalline layers, so as to enable electrical waves to propagate without disturbance.
A method known in the semi-conductor art for obtaining a monocrystalline layer of a material on a substrate is the epitaxial method; to work it requires the crystal lattice constant of the deposited material to be equal or very close to that of the substrate.
That method is not directly applicable in the case of high temperature superconductive materials such as YBaCuO, because their lattice constants differ from those of substrates generally used.
A method, called the hetero-epitaxial method, is also known for depositing a thin layer on a substrate, which method consists in interposing between the substrate and the material to be deposited, a layer of a material having a lattice constant halfway between that of the substrate and that of the material to be deposited.
That method allows a layer to be obtained which is more even than that which would be obtained without the presence of the intermediate layer, but that layer is still too uneven for the intended applications.